Skip performance tests if not in a git repo

As of #12758 (closed) performance tests now compare to the results of the previous commit. Results are stored in git notes. If for some reason the rood directory is not a git repo, we have no results to compare to, nor can we save the new results to git notes. Hence we should skip all performance tests.

Such a use case was encountered in simonpj's workflow where he copies/syslinks the work tree without the .git directory before validating. An open question is can we allow performance testing while not in a git repo or by adjusting the workflow?

Tasks:

When the ghc root directory is not a git repository, skip all performance tests.